The first factor you'll wish to look at is scientific research on a product. Ask the company or salesperson for scientific research on their product.
First of all, if there have been no scientific studies done on the efficacy of the product, be very wary.
If there are scientific studies that show good results from using the product, you should check a very important element in the scientific studies – were they performed using the exact product that you are considering buying?
For example, a salesman may give you a study showing excellent results with “Aloe Vera”. However, the study was most likely performed using fresh aloe vera, not the particular aloe-derived product that you are considering buying. This is very important, because there are many things that a company can do or not do (even without knowing it) in between harvesting the plant and you buying the product that could render the product useless.
That brings us to the second factor to consider, which is processing. How is the product processed? Is it processed in a way that ensures that when you purchase it, it still has all of the nutrients that it should in it?
It can often be difficult to determine if a product is processed properly. So you'll often have to make your decision based on other factors. For example, if you have scientific studies showing that the product gives good results off the shelf, it is apparently processed correctly.